Proton-conducting membrane and use thereof

ABSTRACT

The present invention relates to a novel proton-conducting polymer membrane based on polyazoles which can, because of its excellent chemical and thermal properties, be used in a variety of ways and is particularly useful as polymer electrolyte membrane (PEM) to produce membrane electrode units for PEM fuel cells.

The present invention relates to a novel proton-conducting polymermembrane based on polyazoles which can, because of its excellentchemical and thermal properties, be used in a variety of ways and isparticularly useful as polymer electrolyte membrane (PEM) in PEM fuelcells.

Polyazoles such as polybenzimidazoles (®Celazole) have been known for along time. The preparation of such polybenzimidazoles (PBI) is usuallycarried out by reacting 3,3′,4,4′-tetraminobiphenyl with isophthalicacid or diphenylisophthalic acid or their esters in the melt. Theprepolymer formed solidifies in the reactor and is subsequentlycomminuted mechanically. The pulverulent prepolymer is subsequentlyfully polymerized in a solid-phase polymerization at temperatures of upto 400° C. to give the desired polybenzimidazoles.

To produce polymer films, the PBI is dissolved in polar, aproticsolvents such as dimethylacetamide (DMAc) and a film is produced byclassical methods.

Proton-conducting, i.e. acid-doped, polyazole membranes for use in PEMfuel cells are already known. The basic polyazole films are doped withconcentrated phosphoric acid or sulfuric acid and then act as protonconductors and separators in polymer electrolyte membrane fuel cells(PEM fuel cells).

Due to the excellent properties of the polyazole polymer, such polymerelectrolyte membranes can, when converted into membrane-electrode units(MEEs), be used in fuel cells at continuous operating temperatures above100° C., in particular above 120° C. This high continuous operatingtemperature allows the activity of the catalysts based on noble metalspresent in the membrane-electrode unit (MEE) to be increased.Particularly when using reforming products from hydrocarbons,significant amounts of carbon monoxide are present in the reformer gasand these usually have to be removed by means of a complicated gaswork-up or gas purification. The ability to increase the operatingtemperature makes it possible to tolerate significantly higherconcentrations of CO impurities in long-term operation.

The use of polymer electrolyte membranes based on polyazole polymersenables, firstly, the complicated gas work-up or gas purification to beomitted, at least in part, and, secondly, allows the catalyst loading inthe membrane-electrode unit to be reduced. Both are indispensableprerequisites for wide use of PEM fuel cells since otherwise the costsof a PEM fuel cell system are too high.

The previously known acid-doped polymer membranes based on polyazolesdisplay an advantageous property profile. However, an overallimprovement in these properties has to be achieved in order to be ableto use PEM fuel cells in the intended applications, in particular in theautomobile sector and in decentralized power and heat generation(stationary applications). In addition, the polymer membranes knownhitherto have a high content of dimethylacetamide (DMAc) which cannot beremoved completely by means of known drying methods. The German patentapplication No. 10109829.4 describes a polymer membrane based onpolyazoles in the case of which the DMAc contamination was eliminated.Although such polymer membranes display improved mechanical properties,the specific conductivity does not exceed 0.1 S/cm (at 140° C.).

It is an object of the present invention to provide acid-containingpolymer membranes based on polyazoles which, firstly, have the useadvantages of the polymer membrane based on polyazoles and, secondly,display an increased specific conductivity, in particular at operatingtemperatures above 100° C., and additionally make do withouthumidification of the fuel gas.

We have now found that a proton-conducting membrane based on polyazolescan be obtained when the polyazole prepolymers is fully polymerized inpolyphosphoric acid.

In the case of this novel membrane, the specific after-treatmentdescribed in the German patent application No. 10109829.4 can bedispensed with. The doped polymer membranes display a significantlyimproved proton conductivity and the subsequent doping of the film isdispensed with.

The present invention provides a proton-conducting polymer membranebased on polyazoles which is obtainable by a process comprising thesteps

-   A) Reaction of one or more aromatic tetramino compounds with one or    more aromatic carboxylic acids or esters thereof which contain at    least two acid groups per carboxylic acid monomer, or of one or more    aromatic and/or heteroaromatic diaminocarboxylic acids in the melt    at temperatures of up to 350° C., preferably up to 300° C.,-   B) Dissolution of the solid prepolymer obtained as described in    step A) in polyphosphoric acid,-   C) Heating of the solution obtainable as described in step B) to    temperatures of up to 300° C., preferably up to 280° C., under inert    gas to form the dissolved polyazole polymer,-   D) Formulation of a membrane on a support using the solution of the    polyazole polymer from step C), and-   E) Treatment of the membrane formed in step D) until it is    self-supporting.

The aromatic and heteroaromatic tetramino compounds used according tothe invention are preferably 3,3′,4,4′-tetraminobiphenyl,2,3,5,6-tetraminopyridine, 1,2,4,5-tetraminobenzene,bis(3,4-diaminophenyl) sulfone, bis(3,4-diaminophenyl) ether,3,3′,4,4′-tetraminobenzophenone, 3,3′,4,4′-tetraminodiphenylmethane and3,3′,4,4′-tetraminodiphenyldimethylmethane and also their salts, inparticular their mono-, di-, tri- and tetrahydrochloride derivatives.

The aromatic carboxylic acids used according to the invention aredicarboxylic acids and tricarboxylic acids and tetracarboxylic acids ortheir esters or anhydrides or acid chlorides. The term aromaticcarboxylic acids also encompasses heteroaromatic carboxylic acids. Thearomatic dicarboxylic acids are preferably isophthalic acid,terephthalic acid, phthalic acid, 5-hydroxyisophthalic acid,4-hydroxyisophthalic acid, 2-hydroxyterephthalic acid,5-aminoisophthalic acid, 5-N,N-dimethylaminoisophthalic acid,5-N,N-diethylaminoisophthalic acid, 2,5-dihydroxyterephthalic acid,2,6-dihydroxyisophthalic acid, 4,6-dihydroxyisophthalic acid,2,3-dihydroxyphthalic acid, 2,4-dihydroxyphthalic acid;3,4-dihydroxyphthalic acid, 3-fluorophthalic acid, 5-fluoroisophthalicacid, 2-fluoroterephthalic acid, tetrafluorophthalic acid,tetrafluoroisophthalic acid, tetrafluoroterephthalic acid,1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid,2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid,diphenic acid, 1,8-dihydroxynaphthalene-3,6-dicarboxylic acid,bis(4-carboxyphenyl) ether, benzophenone-4,4′-dicarboxylic acid,bis(4-carboxyphenyl) sulfone, biphenyl-4,4′-dicarboxylic acid,4-trifluoromethylphthalic acid,2,2-bis(4-carboxyphenyl)hexafluoropropane, 4,4′-stilbenedicarboxylicacid, 4-carboxycinnamic acid or their C1–C20-alkyl esters or C5–C12-arylesters or their acid anhydrides or acid chlorides. The aromatictricarboxylic or tetracarboxylic acids and their C1–C20-alkyl esters orC5–C12-aryl esters or their acid anhydrides or acid chlorides arepreferably 1,3,5-benzenetricarboxylic acid (trimesic acid),1,2,4-benzenetricarboxylic acid (trimellitic acid),(2-carboxyphenyl)iminodiacetic acid, 3,5,3′-biphenyltricarboxylic acid,3,5,4′-biphenyltricarboxylic acid.

The aromatic tetracarboxylic acids or their C1–C20-alkyl esters orC5–C12-aryl esters or their acid anhydrides or acid chlorides arepreferably 3,5,3′,5′-biphenyltetracarboxylic acid,1,2,4,5-benzenetetracarboxylic acid, benzophenonetetracarboxylic acid,3,3′,4,4′-biphenyltetracarboxylic acid,2,2′,3,3′-biphenyltetracarboxylic acid,1,2,5,6-naphthalenetetracarboxylic acid,1,4,5,8-naphthalenetetracarboxylic acid.

The heteroaromatic carboxylic acids used according to the invention areheteroaromatic dicarboxylic acids and tricarboxylic acids andtetracarboxylic acids or esters or anhydrides thereof. For the purposesof the present invention, heteroaromatic carboxylic acids are aromaticsystems in which at least one nitrogen, oxygen, sulfur or phosphorusatom is present in the aromatic. Preference is given topyridine-2,5-dicarboxylic acid, pyridine-3,5-dicarboxylic acid,pyridine-2,6-dicarboxylic acid, pyridine-2,4-dicarboxylic acid,4-phenyl-2,5-pyridinedicarboxylic acid, 3,5-pyrazoledicarboxylic acid,2,6-pyrimidinedicarboxylic acid, 2,5-pyrazinedicarboxylic acid,2,4,6-pyridinetricarboxylic acid, benzimidazole-5,6-dicarboxylic acid,and also their C1–C20-alkyl esters or C5–C12-aryl esters or their acidanhydrides or acid chlorides.

The content of tricarboxylic acids or tetracarboxylic acids (based ondicarboxylic acid used) is from 0 to 30 mol %, preferably from 0.1 to 20mol %, in particular from 0.5 to 10 mol %.

The aromatic and heteroaromatic diaminocarboxylic acids used accordingto the invention are preferably diaminobenzoic acid and itsmonohydrochloride and dihydrochloride derivatives.

In step A), preference is given to using mixtures of at least 2different aromatic carboxylic acids. Particular preference is given tousing mixtures comprising aromatic carboxylic acids together withheteroatomic carboxylic acids. The mixing ratio of aromatic carboxylicacids to heteroaromatic carboxylic acids is in the range from 1:99 to99:1, preferably from 1:50 to 50:1.

In particular, these mixtures are mixtures of N-heteroaromaticdicarboxylic acids and aromatic dicarboxylic acids. Nonlimiting examplesare isophthalic acid, terephthalic acid, phthalic acid,2,5-dihydroxyterephthalic acid, 2,6-dihydroxyisophthalic acid,4,6-dihydroxyisophthalic acid, 2,3-dihydroxyphthalic acid,2,4-dihydroxyphthalic acid, 3,4-dihydroxyphthalic acid,1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid,2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid,diphenic acid, 1,8-dihydroxynaphthalene-3,6-dicarboxylic acid,bis(4-carboxyphenyl) ether, benzophenone-4,4′-dicarboxylic acid,bis(4-carboxyphenyl) sulfone, biphenyl-4,4′-dicarboxylic acid,4-trifluoromethylphthalic acid, pyridine-2,5-dicarboxylic acid,pyridine-3,5-dicarboxylic acid, pyridine-2,6-dicarboxylic acid,pyridine-2,4-dicarboxylic acid, 4-phenyl-2,5-pyridinedicarboxylic acid,3,5-pyrazoledicarboxylic acid, 2,6-pyrimidinedicarboxylic acid,2,5-pyrazinedicarboxylic acid.

The prepolymerization in step A) leads, in the selected temperaturerange and when using 3,3′,4,4′-tetraminobiphenyl (TAB) and isophthalicesters (OR), to formation of the corresponding amides or imines (cf. thescheme below)

During the reaction, the prepolymer obtained becomes solid and can, ifdesired after coarse milling, be dissolved in polyphosphoric acid.

The polyphosphoric acid used in step B) is a commercial polyphosphoricacid as is obtainable, for example, from Riedel-de Haen. Thepolyphosphoric acids H_(n+2)P_(n)O_(3n+1) (n>1) usually have an assaycalculated as P₂O₅ (acidimetric) of at least 83%. Instead of a solutionof the prepolymer, it is also possible to produce adispersion/suspension.

The mixture produced in step B) has a weight ratio of polyphosphoricacid to the sum of prepolymer of from 1:10000 to 10000:1, preferablyfrom 1:1000 to 1000:1, in particular from 1:100 to 100:1.

The polyazole-based polymer formed in step C) comprises recurring azoleunits of the formula (I) and/or (II) and/or (III) and/or (IV) and/or (V)and/or (VI) and/or (VII) and/or (VIII) and/or (IX) and/or (X) and/or(XI) and/or (XII) and/or (XIII) and/or (XIV) and/or (XV) and/or (XVI)and/or (XVI) and/or (XVII) and/or (XVIII) and/or (XIX) and/or (XX)and/or (XXI) and/or (XXII)

where

-   Ar are identical or different and are each a tetravalent aromatic or    heteroaromatic group which may have one or more rings,-   Ar¹ are identical or different and are each a divalent aromatic or    heteroaromatic group which may have one or more rings,-   Ar² are identical or different and are each a divalent or trivalent    aromatic or heteroaromatic group which may have one or more rings,-   Ar³ are identical or different and are each a trivalent aromatic or    heteroaromatic group which may have one or more rings,-   Ar⁴ are identical or different and are each a trivalent aromatic or    heteroaromatic group which may have one or more rings,-   Ar⁵ are identical or different and are each a tetravalent aromatic    or heteroaromatic group which may have one or more rings,-   Ar⁶ are identical or different and are each a divalent aromatic or    heteroaromatic group which may have one or more rings,-   Ar⁷ are identical or different and are each a divalent aromatic or    heteroaromatic group which may have one or more rings,-   Ar⁸ are identical or different and are each a trivalent aromatic or    heteroaromatic group which may have one or more rings,-   Ar⁹ are identical or different and are each a divalent or trivalent    or tetravalent aromatic or heteroaromatic group which may have one    or more rings,-   Ar¹⁰ are identical or different and are each a divalent or trivalent    aromatic or heteroaromatic group which may have one or more rings,-   Ar¹¹ are identical or different and are each a divalent aromatic or    heteroaromatic group which may have one or more rings,-   X are identical or different and are each oxygen, sulfur or an amino    group bearing a hydrogen atom, a group having 1–20 carbon atoms,    preferably a branched or unbranched alkyl or alkoxy group, or an    aryl group as further radical,-   R are identical or different and are each hydrogen, an alkyl group    or an aromatic group and    n, m are each an integer greater than or equal to 10, preferably    greater than or equal to 100.

Preferred aromatic or heteroaromatic groups are derived from benzene,naphthalene, biphenyl, diphenyl ether, diphenylmethane,diphenyldimethylmethane, bisphenone, diphenyl sulfone, quinoline,pyridine, bipyridine, pyridazine, pyrimidine, pyrazine, triazine,tetrazine, pyrrole, pyrazole, anthracene, benzopyrrole, benzotriazole,benzooxathiadiazole, benzooxadiazole, benzopyridine, benzopyrazine,benzopyrazidine, benzopyrimidine, benzopyrazine, benzotriazine,indolizine, quinolizine, pyridopyridine, imidazopyrimidine,pyrazinopyrimidine, carbazole, aciridine, phenazine, benzoquinoline,phenoxazine, phenothiazine, acridizine, benzopteridine, phenanthrolineand phenanthrene, which may also be substituted.

Ar¹, Ar⁴, Ar⁶, Ar⁷, Ar⁸, Ar⁹, Ar¹⁰, Ar¹¹ can have any substitutionpattern; in the case of phenylene for example, Ar¹, Ar⁴, Ar⁶, Ar⁷, Ar⁸,Ar⁹, Ar¹⁰, Ar¹¹ can each be ortho-, meta- or para-phenylene.Particularly preferred groups are derived from benzene and biphenyls,which may also be substituted.

Preferred alkyl groups are short-chain alkyl groups having from 1 to 4carbon atoms, e.g. methyl, ethyl, n-propyl or i-propyl and t-butylgroups.

Preferred aromatic groups are phenyl or naphthyl groups. The alkylgroups and the aromatic groups may be substituted.

Preferred substituents are halogen atoms such as fluorine, amino groups,hydroxy groups or short-chain alkyl groups such as methyl or ethylgroups.

Preference is given to polyazoles comprising recurring units of theformula (i) in which the radicals X are identical within a recurringunit.

The polyazoles can in principle also comprise different recurring unitswhich differ, for example, in their radical X. However, they preferablyhave only identical radicals X in a recurring unit.

Further, preferred polyazole polymers are polyimidazoles,polybenzothiazoles, polybenzoxazoles, polyoxadiazoles, polyquinoxalines,polythiadiazoles, poly(pyridines), poly(pyrimidines), andpoly(tetrazapyrenes).

In a further embodiment of the present invention, the polymer comprisingrecurring azole units is a copolymer or a blend comprising at least twounits of the formulae (I) to (XXII) which differ from one another. Thepolymers can be present as block copolymers (diblock, triblock), randomcopolymers, periodic copolymers and/or alternating polymers.

In a particularly preferred embodiment of the present invention, thepolymer comprising recurring azole units is a polyazole which containsonly units of the formula (I) and/or (II).

The number of recurring azole units in the polymer is preferably greaterthan or equal to 10. Particularly preferred polymers have at least 100recurring azole units.

For the purposes of the present invention, polymers comprising recurringbenzimidazole units are preferred. Some examples of the extremelyadvantageous polymers comprising recurring benzimidazole units have thefollowing formulae:

where n and m are each an integer greater than or equal to 10,preferably greater than or equal to 100.

The polyazoles obtainable by means of the process described, inparticular the polybenzimidazoles, have a high molecular weight.Measured as intrinsic viscosity, this is at least 1.4 dl/g and is thussignificantly above that of commercial polybenzimidazole (IV<1.1 dl/g).

If the mixture obtained in step A) comprises tricarboxylic acids ortetracarboxylic acids, branching/crosslinking of the polymer formed isachieved in this way. This contributes to an improvement in themechanical properties.

The formation of the polymer membrane in step D) is carried out by meansof measures (casting, spraying, doctor blade coating) which are knownper se from the prior art for polymer film production. As supports, itis possible to use all supports which are inert under the conditionsemployed. To adjust the viscosity, the solution can, if necessary, beadmixed with phosphoric acid (concentrated phosphoric acid, 85%). Inthis way, the viscosity can be set to the desired value and theformation of the membrane can be made easier. The thickness is from 20to 4000 μm, preferably from 30 to 3500 μm, in particular from 50 to 3000μm.

The membrane produced in step E) is treated at elevated temperatures inthe presence of moisture for a sufficient time until the membrane isself-supporting, so that it can be detached from the support withoutdamage.

The treatment of the membrane in step E) is carried out at temperaturesabove 0° C. and less than 150° C., preferably at temperatures of from10° C. to 120° C., in particular from room temperature (20° C.) to 90°C., in the presence of moisture or water and/or water vapor and/orwater-containing phosphoric acid having a concentration of up to 85%.The treatment is preferably carried out at atmospheric pressure, but canalso be carried out at superatmospheric pressure. It is important forthe treatment to occur in the presence of sufficient moisture so thatthe polyphosphoric acid present contributes to strengthening of themembrane as a result of partial hydrolysis to form low molecular weightpolyphosphoric acid and/or phosphoric acid.

The partial hydrolysis of the polyphosphoric acid in step E) leads tostrengthening of the membrane and to a decrease in the thickness andformation of a membrane having a thickness of from 15 to 3000 μm,preferably from 20 to 2000 μm, in particular from 20 to 1500 nm, whichis self-supporting.

The intramolecular and intermolecular structures (interpenetratingnetworks, IPNs) present in the polyphosphoric acid layer lead to orderedmembrane formation which is responsible for the particular properties ofthe membrane formed.

The upper temperature limit for the treatment in step D) is generally150° C. If moisture is present for an extremely short time, for examplein the case of superheated steam, this steam can also be hotter than150° C. The important factor in determining the upper temperature limitis the duration of the treatment.

The partial hydrolysis (step E) can also be carried out in temperature-and humidity-controlled chambers so that the hydrolysis can becontrolled in a targeted manner in the presence of a defined amount ofmoisture. The amount of moisture can in this case be set in a targetedmanner by means of the temperature or saturation of the environment incontact with the membrane, for example gases such as air, nitrogen,carbon dioxide or other suitable gases, or steam. The treatment time isdependent on the above parameters chosen.

Furthermore, the treatment time is dependent on the thickness of themembrane.

The treatment time is generally in the range from a few seconds to someminutes, for example under the action of superheated steam, or up to anumber of days, for example in air at room temperature and a lowrelative atmospheric humidity. The treatment time is preferably from 10seconds to 300 hours, in particular from 1 minute to 200 hours.

If the partial hydrolysis is carried out at room temperature (20° C.)using ambient air having a relative atmospheric humidity of 40–80%, thetreatment time is from 1 to 200 hours.

The membrane obtained in step E) can be made self-supporting, i.e. itcan be detached from the support without damage and subsequently beprocessed further immediately if desired.

The concentration of phosphoric acid and thus the conductivity of thepolymer membrane according to the invention can be set via the degree ofhydrolysis, i.e. the time, temperature and ambient moisture level.According to the invention, the concentration of phosphoric acid isreported as mole of acid per mole of repeating units of the polymer. Forthe purposes of the present invention, a concentration (mole ofphosphoric acid per repeating unit of polybenzimidazole) of from 10 to50, in particular from 12 to 40, is preferred. Such high degrees ofdoping (concentrations) are very difficult or impossible to achieve bydoping polyazoles with commercially available ortho-phosphoric acid.

Subsequent to the treatment as described in step E), the membrane can becrosslinked on the surface by action of heat in the presence ofatmospheric oxygen. This hardening of the membrane surface effects anadditional improvement in the properties of the membrane.

Crosslinking can also be achieved by action of IR or NIR (IR=infrared,i.e. light having a wavelength of more than 700 nm; NIR=near IR, i.e.light having a wavelength in the range from about 700 to 2000 nm or anenergy in the range from about 0.6 to 1.75 eV). A further method isirradiation with β-rays. The radiation dose is in this case in the rangefrom 5 to 200 kGy.

The polymer membrane of the invention has improved materials propertiescompared to the doped polymer membranes known hitherto. In particular,it displays improved performance compared to known doped polymermembranes. This is due, in particular, to an improved protonconductivity. This is at least 0.1 S/cm, preferably at least 0.11 S/cm,in particular at least 0.12 S/cm, at a temperature of 120° C.

To achieve a further improvement in the use properties, fillers, inparticular proton-conducting fillers, and also additional acids can beadditionally added to the membrane. The addition can be carried outeither during step B), C) or before step D).

Nonlimiting examples of proton-conducting fillers are

-   Sulfates such as CsHSO₄, Fe(SO₄)₂, (NH₄)₃H(SO₄)₂, LiHSO₄, NaHSO₄,    KHSO₄, RbSO₄, LiN₂H₅SO₄, NH₄HSO₄,-   Phosphates such as Zr₃(PO₄)₄, Zr(HPO₄)₂, HZr₂(PO₄)₃, UO₂PO₄.3H₂O,    H₈UO₂PO₄, Ce(HPO₄)₂, Ti(HPO₄)₂, KH₂PO₄, NaH₂PO₄, LiH₂PO₄, NH₄H₂PO₄,    CsH₂PO₄, CaHPO₄, MgHPO₄, HSbP₂O₈, HSb₃P₂O₁₄, H₅Sb₅P₂O₂₀,-   Polyacids such as H₃PW₁₂O₄₀.nH₂O (n=21−29), H₃SiW₁₂O₄₀.nH₂O    (n=21−29), H_(x)WO₃, HSbWO₆, H₃PMo₁₂O₄₀, H₂Sb₄O₁₁, HTaWO₆, HNbO₃,    HTiNbO₅, HTiTaO₅, HSbTeO₆, H₅Ti₄O₉, HSbO₃, H₂MoO₄-   Selenites and arsenides such as (NH₄)₃H(SeO₄)₂, UO₂AsO₄,    (NH₄)₃H(SeO₄)₂, KH₂AsO₄, Cs₃H(SeO₄)₂, Rb₃H(SeO₄)₂,-   Oxides such as Al₂O₃, Sb₂O₅, ThO₂, SnO₂, ZrO₂, MoO₃-   Silicates such as zeolites, zeolites(NH₄+), sheet silicates, network    silicates, H-natrolites, H-mordenites, NH₄-analcines, NH₄-sodalites,    NH₄-gallates, H-montmorillonites-   Acids such as HClO₄, SbF₅-   Fillers such as carbides, in particular SiC, Si₃N₄, fibers, in    particular glass fibers and/or polymer fibers, preferably those    based on polyazoles.

This membrane can also further comprise perfluorinated sulfonic acidadditives (0.1–20% by weight, preferably 0.2–15% by weight, veryparticularly preferably 0.2–10% by weight). These additives lead to aperformance improvement, in the vicinity of the cathode to an increasein the oxygen solubility and oxygen diffusion and to a decrease in theadsorption of phosphoric acid and phosphate on platinum. (Electrolyteadditives for phosphoric acid fuel cells. Gang, Xiao; Hjuler, H. A.;Olsen, C.; Berg, R. W.; Bjerrum, N. J. Chem. Dep. A, Tech. Univ.Denmark, Lyngby, Den. J. Electrochem. Soc. (1993), 140(4), 896–902, andperfluorosulfonimide as an additive in phosphoric acid fuel cell. Razaq,M.; Razaq, A.; Yeager, E.; DesMarteau, Darryl D.; Singh, S. Case Cent.Electrochem. Sci., Case West. Reserve Univ., Cleveland, Ohio, USA. J.Electrochem. Soc. (1989), 136(2), 385–90.)

Nonlimiting examples of persulfonated additives are:

-   Trifluoromethanesulfonic acid, potassium trifluoromethanesulfonate,    sodium trifluoromethanesulfonate, lithium trifluoromethanesulfonate,    ammonium trifluoromethanesulfonate, potassium    perfluorohexanesulfonate, sodium perfluorohexanesulfonate, lithium    perfluorohexanesulfonate, ammonium perfluorohexanesulfonate,    perfluorohexanesulfonic acid, potassium nonafluorobutanesulfonate,    sodium nonafluorobutanesulfonate, lithium nonafluorobutanesulfonate,    ammonium nonafluorobutanesulfonate, cesium    nonafluorobutanesulfonate, triethylammonium    perfluorohexanesulfonate, perfluorosulfonimides and Nafion.

Furthermore, the membrane can further comprise additives which scavenge(primary antioxidants) or destroy (secondary antioxidants) the peroxideradicals generated by reduction of oxygen in operation and therebyincrease the life and stability of the membrane and membrane electrodeunit, as described in JP2001118591 A2. The molecular structures of suchadditives and the way in which they function are described in F. Gugumusin Plastics Additives, Hanser Verlag, 1990; N. S. Allen, M. EdgeFundamentals of Polymer Degradation and Stability, Elsevier, 1992; or H.Zweifel, Stabilization of Polymeric Materials, Springer, 1998.

Nonlimiting examples of such additives are:

-   bis(trifluoromethyl) nitroxide, 2,2-diphenyl-1-picrinylhydrazyl,    phenols, alkylphenols, sterically hindered alkylphenols such as    Irganox, aromatic amines, sterically hindered amines such as    Chimassorb; sterically hindered hydroxylamines, sterically hindered    alkylamines, sterically hindered hydroxylamines, sterically hindered    hydroxylamine ethers, phosphites such as Irgafos, nitrosobenzene,    methyl-2-nitrosopropane, benzophenone, benzaldehyde    tert-butylnitrone, cysteamine, melamines, lead oxides, manganese    oxides, nickel oxides, cobalt oxides.

The possible fields of use of the doped polymer membranes according tothe invention include, inter alia, use in fuel cells, in electrolysis,in capacitors and in battery systems. Owing to their property profile,the doped polymer membranes are preferably used in fuel cells.

The present invention also relates to a membrane-electrode unitcomprising at least one polymer membrane according to the invention. Forfurther information on membrane-electrode units, reference may be madeto the specialist literature, in particular the patents U.S. Pat. Nos.4,191,618, 4,212,714 and 4,333,805. The disclosure in the abovementionedreferences [U.S. Pat. Nos. 4,191,618, 4,212,714 and 4,333,805] regardingthe structure and production of membrane-electrode units and theelectrodes, gas diffusion layers and catalysts to be chosen is herebyincorporated by reference into the present description.

In one variant of the present invention, the membrane can be formeddirectly on the electrode rather than on a support. The treatment instep E) can be shortened in this way, since the membrane no longer hasto be self-supporting. Such a membrane is also subject matter of thepresent invention.

The present invention further provides an electrode provided with aproton-conducting polymer coating based on polyazoles which isobtainable by a process comprising the steps

-   A) Reaction of one or more aromatic tetramino compounds with one or    more aromatic carboxylic acids or esters thereof which contain at    least two acid groups per carboxylic acid monomer, or of one or more    aromatic and/or heteroaromatic diaminocarboxylic acids in the melt    at temperatures of up to 350° C., preferably up to 300° C.,-   B) Dissolution of the solid prepolymer obtained as described in    step A) in polyphosphoric acid,-   C) Heating of the solution obtainable as described in step B) to    temperatures of up to 300° C., preferably 280° C., under inert gas    to form the dissolved polyazole polymer,-   D) Application of a layer to an electrode using the solution of the    polyazole polymer from step C), and-   E) Treatment of the layer formed in step D).

The coating has a thickness of from 2 to 3000 μm, preferably from 3 to2000 μm, in particular from 5 to 1500 μm.

An electrode which has been coated in this way can be incorporated in amembrane-electrode unit which may, if desired, comprise at least onepolymer membrane according to the invention.

General Measurement Methods:

Method of Measuring the IEC

The conductivity of the membrane is dependent to a high degree on thecontent of acid groups expressed by the ion exchange capacity (IEC). Tomeasure the ion exchange capacity, a specimen having a diameter of 3 cmis stamped out and placed in a glass beaker filled with 100 ml of water.The acid liberated is titrated with 0.1 M NaOH. The specimen issubsequently taken out, excess water is dabbed off and the specimen isdried at 160° C. for 4 hours. The dry weight, m₀, is then determinedgravimetrically to a precision of 0.1 mg. The ion exchange capacity isthen calculated from the consumption of 0.1M NaOH to the first titrationend point, V₁ in ml, and the dry weight, m₀ in mg, according to thefollowing formula:IEC=V ₁*300/m ₀Method of Measuring the Specific Conductivity

The specific conductivity is measured by means of impedance spectroscopyin a 4-pole arrangement in the potentiostatic mode using platinumelectrodes (wire, 0.25 mm diameter). The distance between thecurrent-collecting electrodes is 2 cm. The spectrum obtained isevaluated using a simple model consisting of a parallel arrangement ofan ohmic resistance and a capacitor. The specimen cross section of themembrane doped with phosphoric acid is measured immediately beforeinstallation of the specimen. To measure the temperature dependence, themeasurement cell is brought to the desired temperature in an oven andthe temperature is regulated via a Pt-100 resistance thermometerpositioned in the immediate vicinity of the specimen. After the desiredtemperature has been reached, the specimen is held at this temperaturefor 10 minutes before commencement of the measurement.

EXAMPLES

Specimen 1

10 g of prepolymer were placed under a nitrogen atmosphere in athree-neck flask provided with a mechanical stirrer and N₂ inlet andoutlet. 90 g of polyphosphoric acid (83.4±0.5% P₂O₅, as determined byanalysis) were added to the prepolymer. The mixture was firstly heatedto 150° C. and stirred for one hour. The temperature was then increasedto 180° C. for 4 hours, then to 240° C. for 4 hours and finally to 270°C. for 14 hours. At 270° C., 25 g of 85% strength phosphoric acid wereadded to this solution and the mixture was stirred for 1 hour. Thesolution obtained was then cooled to 225° C. to give a still fluidsolution for film casting. This warm solution was applied to a glassplate using a 350 μm doctor blade coater, with the doctor blade coaterand the glass plate having been heated beforehand to 100° C. Themembrane was allowed to stand in air at room temperature (RT=20° C.) for3 days. The polyphosphoric acid attracted moisture from the air and washydrolyzed to phosphoric acid by the moisture absorbed from the air. Theexcess phosphoric acid formed flowed from the membrane. The weight losswas 22% based on the initial weight of the membrane applied by means ofthe doctor blade.

Part of the solution was precipitated after the thermal treatment bymixing with distilled water, filtered, washed three times with distilledwater, neutralized with ammonium hydroxide, then washed three times withdistilled water and finally dried at 120° C. and 1 torr for 16 hours.This gave 2.9 g of PBI powder having an ηinh of 1.47 dl/g measured on a0.4% strength PBI solution in 100 ml of concentrated sulfuric acid(97%).

Specimen 2

10 g of prepolymer were placed under a nitrogen atmosphere in athree-neck flask provided with a mechanical stirrer and N₂ inlet andoutlet. 90 g of polyphosphoric acid (83.4±0.5% P₂O₅, as determined byanalysis) were added to the prepolymer. The mixture was firstly heatedto 150° C. and stirred for one hour. The temperature was then increasedto 180° C. for 4 hours, then to 240° C. for 4 hours and finally to 270°C. for 14 hours. At 270° C., 25 g of 85% strength phosphoric acid wereadded to this solution and the mixture was stirred for 1 hour. Thesolution obtained was then cooled to 240° C. to give a still fluidhomogeneous solution for film casting. This warm solution was applied toa glass plate by means of 350 μm, 700 μm, 930 μm and 1170 μm doctorblade coaters, with the doctor blade coater and the glass plate havingbeen heated beforehand to 100° C. The membrane was allowed to stand inair at RT for 5 days. The polyphosphoric acid attracted moisture fromthe air and was hydrolyzed to phosphoric acid by the moisture absorbedfrom the air. The excess phosphoric acid formed flowed from themembrane. The weight loss of membranes was in the range from 37.5 to 40%based on the initial weight of the membrane applied by means of thedoctor blade. The final thicknesses of the membranes were 210 μm, 376μm, 551 μm and 629 μm.

Part of the solution was precipitated after the thermal treatment bymixing with distilled water, filtered, washed three times with distilledwater, neutralized with ammonium hydroxide, then washed three times withdistilled water and finally dried at 120° C. and 1 torr for 16 hours. Anintrinsic viscosity of ηinh=2.23 dl/g, measured on a 0.4% strength PBIsolution in 100 ml of concentrated sulfuric acid (97%), was obtained forthe PBI powder.

Specimen 3

10 g of prepolymer were placed under a nitrogen atmosphere in athree-neck flask provided with a mechanical stirrer and N₂ inlet andoutlet. 90 g of polyphosphoric acid (83.4±0.5% P₂O₅, as determined byanalysis) were added to the prepolymer. The mixture was firstly heatedto 150° C. and stirred for one hour. The temperature was then increasedto 180° C. for 4 hours, then to 240° C. for 4 hours and finally to 270°C. for 14 hours. At 270° C., 25 g of 85% strength phosphoric acid wereadded to this solution and the mixture was stirred for 1 hour. Thesolution obtained was then cooled to 240° C. to give a still fluidhomogeneous solution for film casting. The warm, 6.5% strength PBIsolution in 104% strength polyphosphoric acid was applied at 200° C. toa glass plate by means of 350 μm, 230 μm, 190 μm and 93 μm doctor bladecoaters, with the doctor blade coater and the glass plate having beenheated beforehand to 100° C. The membrane was allowed to stand in air atRT for 7 days. The polyphosphoric acid attracted moisture from the airand was hydrolyzed to phosphoric acid by the moisture absorbed from theair. The excess phosphoric acid formed flowed from the membrane. Thefinal thicknesses of the membranes were 201 μm, 152 μm, 126 μm and 34μm.

Part of the solution was precipitated after the thermal treatment bymixing with distilled water, filtered, washed three times with distilledwater, neutralized with ammonium hydroxide, then washed three times withdistilled water and finally dried at 120° C. and 1 torr for 16 hours. Anintrinsic viscosity of ηinh=2.6 dl/g, measured on a 0.4% strength PBIsolution in 100 ml of concentrated sulfuric acid (97%), was obtained forthe PBI powder.

In Table 1, the ion exchange capacities and n(H₃PO₄)/n(PBI) values ofspecimens 1–3 are compared with the reference specimen. These values areobtained by titration with 0.1 M NaOH.

TABLE 1 Comparison of ion exchange capacities and n(H₃PO₄)/n(PBI) valuesDesignation of the membrane n(H₃PO₄)/n(PBI) I.E.C. Specimen 1 16.2 157.6Specimen 2 15.0 145.6 Specimen 3 18.7 182.6 Reference specimen forcomparison 9.1 88.5

FIG. 1 shows the temperature-dependent conductivies of specimen 1,specimen 2 and the reference specimen. The temperature-dependentconductivity measurement was carried out using a specially constructed4-pole glass measuring cell. An IM6 impedance spectrometer from ZahnerElektrik was used.

1. A proton-conducting polymer membrane based on polyazoles which isobtained by a process comprising the steps A) Reacting one or morearomatic tetraamino compounds with one or more aromatic carboxylic acidsor esters thereof which contain at least two acid groups per carboxylicacid monomer, or of one or more aromatic and/or heteroaromaticdiaminocarboxylic acids in the melt at temperatures of up to 350° C., B)Dissolution of the solid prepolymer obtained as described in step A) inpolyphosphoric acid, C) Heating of the solution obtained as described instep B) to temperatures of up to 300° C., under inert gas to form thedissolved polyazole polymer, D) Formulating a membrane on a supportusing the solution of the polyazole polymer from step C), and E)Treating the membrane formed in step D) until it is self-supporting. 2.The membrane as claimed in claim 1, wherein step A is conducted at atemperature up to 300° C. and step C is conducted at a temperature up to280° C.
 3. The membrane as claimed in claim 1, wherein said aromatictetraamino compounds used are3,3′,4,4′-tetraaminobiphenyl,2,3,5,6-tetraaminopyridine,1,2,4,5-tetraaminobenzene, bis(3,4-diaminophenyl) sulfone,bis(3,4-diaminophenyl)ether,3,3′,4,4′-tetraaminobenzophenone,3,3′,4,4′-tetraaminodiphenylmethaneand 3,3′,4,4′-tetraaminodiphenyldimethylmethane.
 4. The membrane asclaimed in claim 1, wherein said aromatic dicarboxylic acids used areisophthalic acid, terephthalic acid, phthalic acid,5-hydroxyisophthalicacid,4-hydroxyisophthalic acid, 2-hydroxyterephthalic acid,5-aminoisophthalic acid, 5-N,N-dimethylaminoisophthalic acid, 5-N,N-diethylaminoisophthalic acid, 2,5-dihydroxyterephthalic acid,2,5-dihydroxyisophthalic acid, 2,3-dihydroxyisophthalic acid,2,3-dihydroxyphthalic acid, 2,4-dihydroxyphthalic acid,3,4-dihydroxyphthalic acid, 3-fluorophthalic acid, 5-fluoroisophthalicacid, 2-fluoroterephthalic acid, tetrafluorophthalic acid,tetrafluoroisophthalic acid, tetrafluoroterephthalic acid,1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid,2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid,diphenic acid, 1,8-dihydroxynaphthalene-3,6-dicarboxylic acid,bis(4-carboxyphenyl) ether, benzophenone-4,4′-dicarboxylic acid,bis(4-cargoxyphenyl) sulfone, biphenyl-4,4′-dicarboxylic acid,4-trifluoromethylphthalic acid,2,2-bis(4-carboxyphenyl)hexafluoropropane, 4,4′-stilbenedicarboxylicacid, 4-carboxycinnamic acid or their C1–C20-alkyl esters or C5–C12-arylesters or their acid anhydrides or acid chlorides.
 5. The membrane asclaimed in claim 1, wherein said aromatic carboxylic acids used aretricarboxylic acids, tetracarboxylic acids or their C1–C20-alkyl estersor C5–C12-aryl esters or their acid anhydrides or acid chlorides.
 6. Themembrane as claimed in claim 5, wherein said aromatic carboxylic acidsare 1,3,5-benzene-tricarboxylic acid (trimesic acid);1,2,4-benzenetricarboxylic acid (trimellitic acid);(2-carboxyphenyl)iminodiacetic acid, 3,5,3′-biphenyltricarboxylic acid;3,5,4′-biphenyltricarboxylic acid or 2,4,6-pyridinetricarboxylic acid ormixtures thereof.
 7. The membrane as claimed in claim 1, wherein saidaromatic carboxylic acids used are tetracarboxylic acids, theirC1–C20-alkyl esters or C5 –C12-aryl esters or their acid anhydrides oracid chlorides.
 8. The membranes as claimed in claim 7, wherein saidaromatic carboxylic acids are benzene-1,2,4,5-tetracarboxylic acid;naphthalene-1,4,5,8-tetracarboxylic acid,3,5,3′,5′-biphenyltetracarboxylic acid; benzophenonetetracarboxylicacid, 3,3′,4,4′-biphenyltetracarboxylic acid,2,2′,3,3′-biphenyltetracarboxylic acid,1,2,5,6-naphthalenetetracarboxylic acid, or1,4,5,8,7-naphthalenetetracarboxylic acid.
 9. The membrane as claimed inclaim 5, wherein the content of tricarboxylic acids or tetracarboxylicacids (based on dicarboxylic acid used) is from 0 to 30 mol %.
 10. Themembrane as claimed in claim 5, wherein the content of tricarboxylicacids or tetracarboxylic acids (based on dicarboxylic acid used) is from0.1 to 20 mol %.
 11. The membrane as claimed in claim 5, wherein thecontent of tricarboxylic acids or tetracarboxylic acids (based ondicarboxylic acid used) is from 0.5 to 10 mol %.
 12. The membrane asclaimed in claim 5, wherein heteroaromatic carboxylic acids used areheteroaromatic dicarboxylic acids and tricarboxylic acids andtetracarboxylic acids which have at least one nitrogen, oxygen, sulfuror phosphorus atom in the aromatic, also their C1–C20-alkyl esters orC5–C12-aryl esters or their acid anhydrides or acid chlorides.
 13. Themembrane as claimed in claim 12, wherein said heteroaromatic carboxylicacids are pyridine-2,5-dicarboxylic acid, pyridine-3,5-dicarboxylicacid, pyridine-2,6-dicarboxylic acid, pyridine-2,4-dicarboxylic acid,4-phenyl-2,5-pyridinedicarboxylic acid, 3,5-pyrazoledicarboxylic acid,2,6-pyrimidinedicarboxylic acid, 2,5-pyrazinedicarboxylic acid, or2,4,6-pyridinetricarboxylic acid.
 14. The membrane as claimed in claim1, wherein a polyphosphoric acid having an assay calculated as P₂O₅(acidimetric) of at least 85% is used in step B).
 15. The membrane asclaimed in claim 1, wherein a dispersion/suspension instead of asolution of the prepolymer is produced in step B).
 16. The membrane asclaimed in claim 1, wherein a polyazole-based polymer comprisingrecurring azole units of the formulas (I), (II), (III), (IV), (V), (VI),(VII), (VIII), (IX), (X), (XI), (XII), (XIII), (XIV), (XV), (XVI),(XVII), (XVIII), (XIX), (XX), (XXI) or (XXII) or mixtures thereof

wherein Ar are identical or different and are each a tetravalentaromatic or heteroaromatic group which optionally have one or morerings, Ar₁ are identical or different and are each a divalent aromaticor heteroaromatic group which optionally have one or more rings, Ar₂ areidentical or different and are each a divalent or trivalent aromatic orheteroaromatic group which optionally have one or more rings, Ar₃ areidentical or different and are each a trivalent aromatic orheteroaromatic group which optionally have one or more rings, Ar₄ areidentical or different and are each a trivalent aromatic orheteroaromatic group which optionally have one or more rings, Ar₅ areidentical or different and are each a tetravalent aromatic orheteroaromatic group which optionally have one or more rings, Ar₆ areidentical or different and are each a divalent aromatic orheteroaromatic group which optionally have one or more rings, Ar₇ areidentical or different and are each a divalent aromatic orheteroaromatic group which optionally have one or more rings, Ar₈ areidentical or different and are each a trivalent aromatic orheteroaromatic group which optionally have one or more rings, Ar₉ areidentical or different and are each a divalent or trivalent ortetravalent aromatic or heteroaromatic group which optionally have oneor more rings, Ar₁₀ are identical or different and are each a divalentor trivalent aromatic or heteroaromatic group which optionally have oneor more rings, Ar ₁₁ are identical or different and are each a divalentaromatic or heteroaromatic group which optionally have one or morerings, X are identical or different and are each oxygen, sulfur or anamino group bearing a hydrogen atom, a group having 1–20 carbon atoms, Rare identical or different and are each hydrogen an alkyl group or anaromatic group and n and m are each an integer greater than or equal to10,

where R′are identical or different and are each an alkyl group or anaromatic group and n is an integer greater than or equal to 10, isformed in step C).
 17. The membrane as claimed in claim 16, wherein nend m are each an integer greater than
 100. 18. The membrane as claimedin claim 1, wherein a polymer selected from the group consisting ofpolybenzimidazole, poly(pyridines), poly(pyrimidines), polyimidazoles,polybenzothiazoles, polybenzoxazoles, polyoxadiazoles, polyquinoxalines,polythiadiazoles and poly(tetrazapyrenes) is formed in step C).
 19. Amembrane as claimed in claim 1, wherein the polymer formed in step C)comprises recurring benzimidazole units of the formula

wherein n and m are each an integer greater than or equal to
 10. 20. Themembrane as claimed in claim 19 wherein n and m are each an integergreater than or equal to
 100. 21. The membrane as claimed in claim 1,wherein the viscosity is adjusted by addition of phosphoric acid afterstep C) and before step D).
 22. The membrane as claimed in claim 1,wherein the membrane produced in step D) is treated in the presence ofmoisture at temperatures and for a time until the membrane isself-supporting and can be detached from the support without damage. 23.The membrane as claimed in claim 1, wherein the treatment of themembrane in step E) is carried out at temperatures of from 0° C. to 150°C. in the presence of moisture or water and/or water vapor.
 24. Themembrane as claimed in claim 1, wherein the treatment of the membrane instep E) is carried out at temperatures of from 10° C. to 120° C. in thepresence of moisture or water and/or water vapor.
 25. The membrane asclaimed in claim 1, wherein the treatment of the membrane in step E) iscarried out at temperatures of from 20° C. to 90° C. in the presence ofmoisture or water and/or water vapor.
 26. The membrane as claimed inclaim 1, wherein the treatment of the membrane in step E) is carried outfor from 10 seconds to 300 hours.
 27. The membrane as claimed in claim1, wherein the treatment of the membrane in step E) is carried out forfrom 1 minute to 200 hours.
 28. The membrane as claimed in claim 1,wherein an electrode is chosen as support in step D) and the treatmentin step E) is such that the membrane formed is no longerself-supporting.
 29. The membrane as claimed in claim 1, wherein themembrane formed in step D) has a thickness of from 20 to 4000 μm. 30.The membrane as claimed In claim 1, wherein the membrane formed in stepE) has a thickness of from 15 to 3000 μm and is self-supporting.
 31. Themembrane as claimed in claim 1, wherein the membrane formed in step E)has a thickness of from 20 to 2000 μm, and is self-supporting and themembrane formed in step D) has a thickness of from 30 to 3500 μm. 32.The membrane as claimed in claim 1, wherein the membrane formed in stepE) has a thickness of from 20 to 1500 μm, and is self-supporting and themembrane formed in step D) has a thickness of from 50 to 3000 μm.
 33. Anelectrode provided with a proton-conducting polymer coating based onpolyazoles which is obtained by a process comprising the steps A)Reacting one or more aromatic tetraamino compounds with one or morearomatic carboxylic acids or esters thereof which contain at least twoacid groups per carboxylic acid monomer, or of one or morn aromaticand/or heteroaromatic diaminocarboxylic acids in the melt attemperatures of up to 350° C., B) Dissolution of the solid prepolymerobtained as described in step A) in polyphosphoric acid, C) Heating ofthe solution obtained as described in step B) to temperatures of up to300°C., under inert gas to form the dissolved polyazole polymer, D)Application of a layer to an electrode using the solution of thepolyazole polymer from step C), and E) Treating the layer formed in stepD).
 34. The electrode as claimed in claim 33, wherein step A isconducted at temperatures up to 300° C. and step C is conducted attemperatures up to 280° C.
 35. An electrode as claimed in claim 33,wherein the coating has a thickness of from 2 to 3000 μm.
 36. Anelectrode as claimed in claim 35, wherein the coating has a thickness offrom 3 to 2000 μm.
 37. An electrode as claimed in claim 33, wherein thecoating has a thickness of from 5 to 1500 μm.
 38. A membrane-electrodeunit comprising at least one electrode and at least one membrane asclaimed in claim
 1. 39. A membrane-electrode unit comprising at leastone electrode as claimed in claim 35 and at least one membrane.
 40. Afuel cell comprising one or more membrane-electrode units as claimed inclaim 39.